1. Field of the Invention
The present invention relates to an electrophotographic color toner and particularly to an electrophotographic color toner, which is prepared as a mixture of an achromatic white toner and a chromatic color toner containing at least one member selected from yellow, magenta, cyan, etc. and which has the configuration for enlarging a color reproduction region of the color toner and stabilizing developing characteristic of the color toner.
2. Description of the Related Art
An electrophotography is a technique widely used in an image forming apparatus such as a copying machine, an electrophotographic facsimile machine or an electrophotographic printer. A method using a photoconductive electrical insulator is generally used as the electrophotography (U.S. Pat. No. 2,297,691).
This method is carried out as follows. Light emitted from a laser, an LED, etc. is applied onto a photoconductive electrical insulator charged by corona discharge or by a charge-supply roller to thereby form an electrostatic latent image. Then, resin powder colored by pigment or dye and called “toner” is electrostatically deposited on the electrostatic latent image and developed to thereby form a visualized toner image. Then, the toner image is transferred onto a recording medium such as a sheet of paper or a film.
On this occasion, it is necessary to fix the toner image onto the recording medium because the toner image is an image of powder merely put on the recording medium.
Therefore, in a final step, the toner deposited on the recording medium is melted by heat, pressure, light or the like and then solidified to thereby finally obtain a toner image fixed onto the recording medium.
As described above, fixing of toner is performed in such a manner that toner, which is powder containing a thermoplastic resin (hereinafter referred to as binder resin) as a main component, is melted by heat and fixed onto the recording medium. As methods for fixing toner, there are commonly known a heat roll method using rollers for directly heating and pressurizing the recording medium having the toner image formed thereon, and a flash fixing method for fixing toner onto the recording medium by flash light emitted from an xenon flash lamp or the like.
To obtain a color image, there are known a printing method for performing color printing by developing and superposing three kinds of color toner, namely, yellow toner, magenta toner and cyan toner or four kinds of color toner inclusive of black toner in addition to the three kinds of color toner, and a printing method for performing color printing by superposing at least two kinds of black or color toner (JP-A-Sho.61-132959 and U.S. Pat. No. 4,699,863).
In the former printing method, the four kinds of color toner, namely, yellow toner, magenta toner, cyan toner and black toner, are set in an yellow developing unit, a magenta developing unit, a cyan developing unit and a black developing unit respectively. When the respective kinds of color toner are developed, a print image is formed.
In this case, if the developing condition is optimized, developing characteristic can be kept constant even when the respective kinds of color toner are different in physical solid-state properties. The apparatus is however complex in structure, so that the cost of the apparatus becomes high.
In the latter printing method, color printing can be performed if at least one developing unit can be provided for developing color toner. The apparatus is simple in structure, so that the cost of the apparatus becomes low.
As an apparatus using the latter printing method, there is known a printing apparatus having a fluidized bed in which kinds of toner substantially equal in physical properties but different in color are mixed homogeneously at a predetermined ratio (JP-A-Hei.6-348101 and U.S. Pat. No. 5,866,286).
Heretofore, electrical resistivity of toner has been adjusted by a method of adding an electrically conductive additive to the toner. In a two-component developing agent containing toner and a magnetic carrier, electrical resistivity can be controlled when electrical conductivity of a core material or a coating material of the magnetic carrier combined with the toner is adjusted.
For example, a method of adding an electrically conductive additive to toner is known as a method for controlling electrical resistivity of the toner (JP-A-Hei.5-19525, JP-A-Hei.11-327192, and U.S. Pat. No. 6,165,666).
This is a technique for internally or externally adding an electrically conductive additive to color toner to thereby change electrical resistivity of the color toner.
When at least two kinds of toners selected from yellow toner, magenta toner, cyan toner, black toner, etc. are mixed in this manner, color toner having any color can be provided.
To perform color printing by the latter printing method, various kinds of color toner need to be prepared in accordance with various print colors. To obtain various kinds of color toner, colorants of various materials need to be used.
When the material of the colorant varies, electrical resistivity of color toner, however, varies according to electrical resistivity of the colorant. For this reason, whenever color toner is provided, it is necessary to control electrical resistivity of the color toner accurately in order to use the color toner in a developing unit in the same condition. In order to control electrical resistivity accurately, it is necessary to repeat enormous experimentation and evaluation. There is a problem that it is actually impossible to provide various kinds of color toner.
Even if various kinds of color toner were provided by using colorants changed by the aforementioned method, it is necessary to clean piping and production equipment whenever a color toner product different in color is to be produced because diversified color toner products must be produced. There is a problem that a large demerit occurs in production cost or the like.
That is, though various kinds of color toner can be provided when at least two kinds of toner selected from yellow toner, magenta toner and cyan toner are mixed, the yellow toner, the magenta toner and the cyan toner are different in electrical resistivity because a yellow pigment, a magenta pigment and a cyan pigment used as colorants in the yellow toner, the magenta toner and the cyan toner respectively are widely different in electrical resistivity.
If color toner obtained by combination/mixing of these kinds of color toner different in electrical resistivity is printed, these kinds of color toner combined are developed unevenly. There is a problem that the color tone of a print image becomes unstable.
For this reason, there is required a method for accurately adjusting electrical resistivities of yellow toner, magenta toner and cyan toner in which a yellow pigment, a magenta pigment and a cyan pigment different in electrical resistivity are contained respectively.
The color that can be reproduced by color toner prepared by mixing of yellow toner, magenta toner and cyan toner is limited to a predetermined range in an L*a*b* color space. This situation will be described with reference to FIG. 9.
Refer to FIG. 9.
That is, though it is possible to reproduce any color on a closed curve Y-L-A-B-M-D-E-F-C-G-H-J-Y shown in FIG. 9 when the mixture ratio of these kinds of toner is changed, it is impossible to reproduce any other color in the inside of the curve and any other color different in terms of a lightness axis L* perpendicular to an a*-b* plane.
Therefore, an improved color adjusting method needs to be provided in order to reproduce other colors than the colors taken on the closed curve Y-L-A-B-M-D-E-F-C-G-H-J-Y.